Biochemical and immunochemical studies of proteins of human and animal lenses and cataracts have been undertaken. An intrinsic protein of the membranes of the human lens was found to be similar to that of the animal lenses. However, with aging a second membrane protein emerges in the human lens. This protein apparently is derived from the principal membrane protein. As the cataract develops major changes in the lens proteins occur. Aggregation of proteins appears to be an important factor. This process leads to the formation of soluble, heavy molecular weight proteins and also insoluble proteins. The conversion of soluble proteins to insoluble proteins appears as a constant finding in most cataracts. Degradation and leakage of proteins in the lens also occur during cataract formation.